1. Field of Invention
This invention is directed to the field of semiconductor crystal growing and, more particularly, to an apparatus for weighing crystals during Czochralski crystal growing processes.
2. Description of Related Art
The Czochralski crystal growing technique is commonly used to commercially grow bulk semiconductor single crystals such as silicon crystals. In this technique, molten semiconductor material is contained in a crucible in a chamber of a crystal growing apparatus under controlled temperature and pressure conditions. A seed crystal is lowered into contact with the melt and then slowly raised to pull a single crystal from the melt. Typically, the single crystal is rotated during pulling. The grown single crystal includes neck, shoulder, body and tail portions formed in this respective order. The body is used to produce semiconductor wafers.
The body portion of single crystals is required to be of uniform high quality. To achieve such high quality, it is necessary to accurately control the diameter of the body portion during crystal growth to enable the homogeneity of the grown crystal to be accurately controlled. The concentration of deliberately added impurities and stoichiometry are sensitive to growth rate fluctuations. It is desirable to have a uniform cross-section along the length of the body portion of the crystal to provide maximum volume of crystal for subsequent slicing operations to form wafers.
In addition to properly forming the body portion of single crystals, it is also important to properly form the tail portion in order to produce body portions of high quality and sufficient length, and also to efficiently utilize semiconductor material. Known methods of Czochralski crystal growth include weighing methods that measure the weight of grown crystal to determine the completion time of the body portion and, accordingly, the time at which to start forming the tail portion which is adjacent to the body portion and the lowermost portion of the single crystal. To form the tail portion, the pulling rate of the single crystal and/or the temperature of the melt are varied to gradually decrease the single crystal diameter so that the tail portion has a tapered configuration. The tail portion typically has a generally conical shape.
As stated, in order to form the tail portion, the pulling rate and the melt temperature are adjusted at a selected time during crystal growing. It is important that the formation of the tail portion be started at the correct time, and not too early or too late in the crystal growing process. If the tail portion is started too early, although the tail portion may be properly formed, there may be an excessive amount of residual melt in the crucible when the tail portion is completed. This residual melt is expensive waste material due to the high cost of semiconductor material. In addition, the excessive residual melt does not go into forming the body portion of the single crystal, and, consequently, the body portion may be undersized. As a result, the number of wafers that can be formed from the body portion is reduced.
If, on the other hand, the formation of the tail portion is started too late during the crystal growing process, then the tail portion may not be fully formed. In such instances, the entire amount of melt in the crucible goes into the crystal before the tail portion is completed. As a result, it is typically necessary to remove more than a desired volume of material from the lower part of the body portion adjacent to the tail portion to remove crystal that tends to have poor crystal structure and, consequently, also poor electrical properties. Thus, starting the formation of the tail portion too late during crystal growing also causes waste of semiconductor material and a reduction in the number of wafers that can be formed from single crystals.
Known apparatuses for weighing single crystals during Czochralski crystal growing processes do not provide sufficiently accurate measurements of crystal weight to enable the completion of the formation of the body portion to be accurately determined. Particularly, these apparatuses can give weight measurements of grown crystal that vary depending on pressure and temperature conditions present in the crystal growing vessels. Consequently, tail portions tend to be started either too early or too late during the crystal growing process because the completion of formation of the body portion is not accurately determined, resulting in the above-described problems.
Thus, there is a need for an apparatus that can accurately weigh grown single crystals during Czochralski crystal growing processes to enable the production of properly formed single crystals.